anchor ice

Anchor ice forms in freshwater like a second skin over the frozen bottoms of chilled rivers and streams. Cold bottom water slows and then pools behind stones, initiating its formation. Eventually, the spread of ice may bind rocks, plants, invertebrates, and other organisms together in sheets, pinning them to the streambed. In a similar fashion, anchor ice may coat submerged structures or objects—pier footings and boat anchors—in cold, motionless water. Anchor ice also forms on the ﬂoors of polar seas, where it is known as anag¨lu to Iñupiaq Eskimos. When the mass of this type of anchor ice is sufﬁcient, it will suddenly break loose in jagged fragments and rise to the surface. (Exuding its salts diminishes the speciﬁc gravity of sea ice; when a section of it grows large enough for the force pushing it toward the surface to overcome the strength of its anchor hold, it rises.) In like fashion, when strong currents or storms churn the upper layers of water, the agitation might cause spears and blocks of anchor ice, with their load of embedded seaweed, abrasive sand, rocks, and shells, to break free. Such unexpected surfacing of huge ice chunks can upend, puncture, or damage a small boat. Other names include depth, underwater, and lappered ice